Various means of freezing particulate materials have been known and utilized in the refrigeration industry. Typical refrigerated freezers using mechanical refrigeration operated on fluorcarbon refrigerants have been used for what is termed normal slow freezing of particulate materials, such as foodstuffs. Mechanically refrigerated freezers typically have the drawback that temperatures are not sufficiently low, such that delicate foodstuffs, having a significant water content, are frozen in a well preserved structural manner wherein structural membranes remain intact. Additionally, such mechanically refrigerated freezers tend to allow the particulate materials to agglomerate during the freezing process due to surface moisture which is frozen on the individual particles.
Subsequently, various cryogenic freezers have been designed and utilized by the industry, wherein the cryogenic freezers utilize cryogenic fluids to directly impart the refrigeration value to the particulate material to be frozen. Typical cryogenic fluids include liquid nitrogen, liquid carbon dioxide, liquid air and various inert halogenated hydrocarbons. In a cryogenic freezer, the cryogenic fluid directly contacts the particulate materials to be frozen, which is in contrast to mechanical refrigerated freezers wherein a liquid cryogen may be utilized as a heat transfer media, but is maintained in a separate distinct circuit with only indirect heat exchange with the product to be frozen.
In the known cryogenic freezers, the particulate material to be frozen is typically immersed in the cryogenic liquid and is rapidly frozen so as to avoid destruction of any structure or membranes of the particulate material and to avoid the agglomeration of individual particles of the material to be frozen. However, during the course of the introduction of relatively warm particulate materials into the bath of cryogenic liquid, the heat removed from the particulate materials is imparted to the cryogenic liquid with the subsequent vaporization of liquid to a cryogenic vapor. This vapor is not without value. Most cryogenic freezers utilize this still significantly cold cryogenic vapor for additional freezing duty by direct contact of the vapor with incoming particulate material as a precooling step or with subsequent contact of the immersed initially frozen material with cryogenic vapors to further reduce the temperature of the particulate material. However, the production of cryogenic vapor is not without drawbacks. Typically, the cryogenic vapor, if it is not produced from liquified air, constitutes a health hazard to personnel working around the cryogenic freezer. In light of the necessity for apertures into and out of the freezer so as to introduce particulate material for freezing and removing frozen particulate material as product, the cryogenic vapors produced during freezing operations are usually capable of escaping into the adjacent space around the freezer where potentially adverse effects of the vapors would be experienced by operating personnel. Where the cryogenic vapor is nitrogen or carbon dioxide, any sizable amount of cryogenic vapor escaping into the atmosphere surrounding the freezer would create a potential for asphyxiation of operating personnel.
It has been known to remove such excess cryogenic vapors by various ducting and exhaust techniques. In U.S. Pat. No. 3,345,828 and U.S. Pat. No. Re. 28,712, a system is disclosed wherein a thermocouple 39 located near one end of a conveyor belt senses temperature produced by the cooling cryogenic vapor and provides an input into an automatically operated blade 76 at the end of the cryogenic freezer. The blade controls the amount of cryogenic vapor that exits the freezer, such that sufficient vapor remains in the freezer inner space so as to maintain a cold vapor bath commensurate with the location of the thermocouple. Cryogenic vapor which exits along stream E passed the blade 76 is removed from the space adjacent the freezer by conduit 51 exhaust fan 52 and additional conduit 54. Therefore, in these patents, the depth of cryogenic vapor is sensed and a blade which controls the exit of gas to the space outside the freezer is adjusted to control that exiting flow; which flow is then subsequently intercepted by an exhaust system which is not controlled by the thermocouple or by the temperature sensing means within the freezer itself.
In U.S. Pat. No. 3,403,527, a cryogenic vapor freezer utilizing a conveyor belt is set forth wherein any escaping cryogenic vapors are removed by exhaust collector 43 and conduit 45 powered by an exhaust fan. Control of the exhaust rate based upon freezer conditions is not set forth.
In U.S. Pat. No. 3,892,104, a cryogenic freezer utilizing cryogenic vapor as a cooling medium provides for pressure sensing in the supply line of cryogenic liquid to be vaporized in the freezer for the basis of a signal to control internal circulation fans 74, such that circulation of cryogenic vapor within the freezer is commensurate with liquid flow to a spray header 35.
Finally in U.S. Pat. No. 3,926,080 an immersion cryogenic freezer is set forth wherein foodstuffs are conveyed through a bath for surface freezing prior to further processing of the foodstuff in downstream sectioning and cutting apparatus. No control of cryogenic vapor dispersal is set forth.
The prior art has suggested various methods for control of vapors emanating from a cryogenic freezer. However, none of the prior art suggests a mode for controllably removing cryogenic vapors from a cryogenic freezer, nor do they provide for termination of the exhaust mode if an influx of ambient outside air occurs into the freezer. Additionally, the direct automatic control of an exhaust fan as in the present invention provides for a linear increase in gas flow commensurate with the fan speed related to the electrical output of the sensing means. This is in contrast with the nonlinear gas flow commensurate with an altering damper position as recited in U.S. Pat. No. 3,345,828. Finally, the prior art typically does not have automatic control of the exhaust fan associated with the respective cryogenic freezers, but rather requires the exhaust fan to be manually operated, or operated continuously.
The attributes of the present invention will be more fully appreciated from the disclosure which follows.